1. Field of the Invention
The present invention relates to a feeder device used in an image forming or document feeding apparatus such as printer and facsimile apparatus, for feeding stacked media sheets one after another along a feed path.
2. Discussion of Related Art
Conventionally, a document feeding apparatus such as printer and facsimile apparatus is equipped with a sheet feeder as a feeding device in which a plurality of recording paper sheets as media sheets stacked on a sheet supply tray are separated one by one from the supply tray, so as to fed along a feed path.
In the sheet feeder, the paper sheets are fed one after another along the feed path, by rotation of a sheet supply roller that is held in contact with an uppermost one of the paper sheets stacked on the sheet supply tray. Commonly, a DC motor is used as a drive source for driving the sheet supply roller, so that the sheet supply roller is rotated with the DC motor being driven in response to a drive command that is supplied to the DC motor.
The DC motor inherently has an advantage that its operating noise is relatively low. However, the DC motor tends to suffer from fluctuation in its rotational velocity that could be caused by, for example, disturbance such as electromagnetic wave and fluctuation in load acting on the sheet supply roller. Therefore, the sheet feeder is equipped with a feedback control system for supplying a variable that is suitably determined by comparing a detected value of a rotational velocity of the sheet supply roller with a reference value of the rotational velocity of the sheet supply roller, so that the DC motor can be controlled based on the suitable variable.
In the conventional sheet feeder as described above, it is determined whether jamming or other sheet feed error occurs by monitoring change in output signals supplied from sensors such as feed sensor and register sensor that are disposed in suitable portions of the feed path along which the paper sheet is to be fed.
Further, JP-2003-312893A discloses a sheet feeder that is equipped with a detector for detecting jamming or other sheet feed error by comparing a monitored value of an electric current supplied to a DC motor with a predetermined threshold value of the supplied electric current. This arrangement is based on a fact that the electric current supplied to the DC motor is increased with increase of a load acting on the DC motor, which increase could be caused by reduction or stop of rotation of a sheet supply roller, for example, in event of occurrence of jamming or other sheet feed error.
The load acting on the DC motor is fluctuated by a friction acting between the sheet supply roller and the recording paper sheet that are in contact with each other. An amount of this friction varies depending upon a kind or type of the paper sheet. For example, the load acting on the DC motor is larger when a paper sheet having larger thickness and weight is supplied, than when a paper sheet having smaller thickness and weight is supplied. Further, a study revealed that the load varies depending upon whether the paper sheet is a glossy paper or a standard paper, and that load is larger in supply of the glossy paper than in supply of the standard paper even if the glossy paper and the standard paper are the same with respect to thickness. However, in the sheet feeder disclosed in the above-identified published document, the variation of the load depending upon the type of the paper sheet is not taken into account, so that there is a possibility of erroneous determination of sheet feed error, depending on the type of the paper sheet accommodated in a sheet supplying cassette, even when the recording paper sheet is being normally fed.
Further, in the disclosed sheet feeder, it is possible to detect substantially three kinds of error statuses such as absence of the paper sheet, feed failure due to slip motion of the sheet supply roller, and paper jamming. However, there has been a need for detection of other kind of error status.